The invention relates to the field of optical fiber power amplifier and laser systems used for optical communications, and more particularly to optical fiber pumping apparatus and method for use with a double-cladding fiber in an optical signal amplifier or laser configuration.
Over the past years, there has been a constant need to increase the output power of fiber amplifiers and lasers to comply with requirements of modern electronic communication techniques, such as Dense Wavelength Division Multiplexing (DWDM) optical communication, wherein multiple data channels share the available amplifier output power for providing simultaneous channels amplification. Optical Inter-Satellite Links (OISL) is another growing application wherein a diffraction-limited beam has to be emitted in free-space and received thousands of kilometers away. In the latter case, propagation distances involved prescribe high-power, and the diffraction-limited quality of the optical beam requires the use of single-mode optical fiber amplifiers. For similar reasons, some particular applications of Light Detection And Ranging (LIDAR) require high-power fiber lasers.
The power output of fiber lasers and amplifiers is directly related to the absorbed pump power in the amplifying, rare-earth-doped fiber section, and thus it is related to the amount of pump power that can be coupled to a single fiber. The amplified signal has to be transversally single-mode in order to have stable amplification and diffraction-limited output with high output power. Thus, the amplification doped region must be confined to a single-mode core. For optical amplification to occur, the pump must overlap with the signal in this single-mode doped core. The coupling of a pump signal into a single-mode core may be performed with a small area laser diode. In practice, the diode active area must be smaller than the diameter of the single-mode core to allow an efficient coupling. Reducing the pump diode active area limits its output power proportionally, which in turn limits the output power of the fiber amplifier. A known way to get around this limitation in an optical fiber amplifier application is disclosed in U.S. Pat. No 5,721,636 to Erfogan et al., which consists in providing a linear array of pump lasers coupled to a plurality of series connected amplifier fiber sections using a coupler an a plurality of routers. Another pumping approach applied to a Nd:YAG laser is taught by Fan et al. in a paper entitled xe2x80x9cScalable, end-pump, diode-laser-pumped laserxe2x80x9d Optics Letters, Vol. 14, No. 19, October 1989, pp. 1057-1059. In that paper, there is proposed to couple a multiple-diode pump source to the Nd:YAG laser, wherein the beam generated by each laser diode is collimated individually to be directed to a large focusing lens providing pump energy injection into the Nd:YAG laser. Another known way to increase the pump power of optical fiber lasers and amplifiers consists in using a double-cladding rare-earth doped fiber. The high-power, broad beam pump radiation generated by the laser diode signal is coupled to a larger multi-mode inner cladding inside which a rare-earth doped single-mode core of a higher index of refraction is contained. The inner cladding is usually surrounded by an outer cladding having a lower index of refraction to prevent radiation from propagating out of the inner cladding. Such known optical fiber structure is disclosed in U.S. Pat. No. 4,815,079 issued to Snitzer et al., which also discloses side coupling configurations for pumping of radiation into the inner cladding. In a typical optical fiber application, an input signal injected within the core is amplified through pump energy transfer from the inner cladding, while in a typical optical fiber laser application, a pair of spaced grating reflectors integrated within the receiving fiber forms an optical resonating cavity allowing the pump energy to be converted to a power laser beam. Many configurations for double-cladding fiber amplifiers or lasers, with many pump/signal multiplexing/demultiplexing approaches are disclosed by Goldberg et al. in the paper entitled xe2x80x9cHigh-efficiency side-coupling of light into double-cladding fibers using imbedded V-groovesxe2x80x9d, OFC""96 technical Digest, 1996, pp. 91-92, in international PCT application published under No. WO 95/10868 naming Gapontsev et al. as inventors, in U.S. Pat. No. 5,790,722 issued to Minden et al. and in U.S. Pat. No. 5,659,644 to DiGiovanni et al. Although those prior references teach the use of a double-cladding fiber, all disclose the use of a single pump source, thus limiting the pump power that could be transferred to the core via a side coupling with the inner cladding, such as taught by Goldberg et al. and Gapontsev et al., or via an end coupling with the inner cladding, such as taught by Minden et al. and DiGiovanni et al.
In order to further increase the output power, it is also known to use laser diode linear arrays as disclosed in U.S. Pat. No. 5,268,978 issued to Po et al. and in U.S. Pat. No. 5,533,163 issued to Muendel, wherein particular optical coupling devices are proposed for combining the individual laser beams of the diodes included in the array into a single output beam to be pumped into the inner cladding of a double-cladding optical fiber laser. Although representing improvements over prior pump devices using conventional fibers, the pump power that can be transferred to the double-cladding fiber with those multi-pump source devices is inherently limited to the number of laser diodes that can be physically integrated in the linear array.
Moreover, apart from fiber manufacturing techniques, the multiplexing/demultiplexing of the pump and the signal is an important challenge associated with the double-cladding fiber amplifier configuration. Since the signal is to be injected in or extracted from the single-mode inner core and the pump power is to be injected in the multi-mode inner cladding closely surrounding the core, traditional fusion fiber coupling to one end of the amplification medium is no more possible. There is still a need for improved optical pumping fiber techniques and devices that can further increase the pump power that can be transferred to a double-cladding fiber in amplifier/laser applications, while allowing reliable and efficient input signal coupling when used in fiber amplifier applications.
It is an object of the present invention to provide apparatus and method for optical pumping of a double-cladding fiber in amplifier or laser configuration which are simple and reliable, while exhibiting high pump power rating.
It is another object of the present invention to provide apparatus and method for optical pumping of a double-cladding fiber in an amplifier configuration that allow simultaneous efficient input signal coupling.
According to one or both of the above objects, from a broad aspect of the present invention, there is provided an optical fiber pumping apparatus for use with a double-cladding fiber in an optical amplifier or laser configuration, said fiber including a single-mode core doped with an active material and disposed within an inner multi-mode cladding surrounded by an outer cladding, said inner cladding being provided with a pumping input portion located at an input end of the inner cladding. The apparatus comprises a plurality of pump sources disposed in a spatial [configuration] arrangement for radiating pump energy along an optical axis through a surrounding generally annular area while leaving a middle area for one of an input optical signal injection into the core and an output optical signal extraction from the core, and an optical coupling device having an optical input portion generally aligned with the optical axis to collect the pump energy and having an optical output portion aligned with the pumping input portion for transferring the pump energy to the inner cladding.
From another broad aspect of the present invention, there is provided an optical fiber pumping apparatus for use with a double-cladding fiber length in an optical amplifier or laser configuration, said fiber including a single-mode core doped with an active material and disposed within an inner multi-mode cladding surrounded by an outer cladding, said inner cladding being provided with first and second spaced pumping input portions located at opposed first and second input ends of the inner cladding. The apparatus comprises a first set of pump sources disposed in a first spatial [configuration] arrangement for radiating pump energy along a first optical axis through a surrounding generally annular area while leaving a middle area for one of an input optical signal injection into the core and an output optical signal extraction from the core and a first optical coupling device having an optical input portion generally aligned with the first optical axis to collect the pump energy, and having an optical output portion aligned with the first pumping input portion for transferring the pump energy to the inner cladding. The apparatus further comprises a second set of pump sources disposed in a second spatial [configuration] arrangement for radiating further pump energy along a second optical axis through a surrounding generally annular area and a second optical coupling device having an optical input portion generally aligned with the second optical axis to collect the further pump energy, and having an optical output portion aligned with the second pumping input portion for transferring the further pump energy to the inner cladding.
From another broad aspect of the present invention. there is provided a pumped optical fiber system comprising a double-cladding fiber in one of an optical signal amplifier and laser configuration, said fiber including a single mode core doped with an active material and disposed within an inner multi-mode cladding surrounded by an outer cladding, said inner cladding being provided with a pumping input portion located at an input end of the inner cladding. The apparatus further comprises a plurality of pump sources disposed in a spatial arrangement for radiating pump energy along a optical axis through a surrounding generally annular area while leaving a middle area for one of an input optical signal injection into the core and an output optical signal extraction from the core, and an optical coupling device having an optical input portion generally aligned with the optical axis to collect the pump energy and having an optical output portion aligned with the pumping input portion for transferring the pump energy to the Inner cladding.
From another broad aspect of the present invention, there is provided a pumped optical fiber system comprising a double-cladding fiber in one of an optical signal amplifier and laser configuration, said fiber including a single mode core doped with an active material and disposed within an inner multi-mode cladding surrounded by an outer cladding, said inner cladding being provided with first and second spaced pumping input portions located at opposed first and second input ends of the inner cladding. The apparatus further comprises a first set of pump sources disposed in a first spatial arrangement for radiating pump energy along a first optical axis through a surrounding generally annular area while leaving a middle area for one of an input optical signal injection into the core and an output optical signal extraction from the core, and a first optical coupling device having an optical input portion generally aligned with the first optical axis to collect the pump energy and having an optical output portion aligned with the first pumping input portion for transferring the pump energy to the inner cladding. The apparatus further comprises a second set of pump sources disposed in a second spatial arrangement for radiating further pump energy along a second optical axis through a surrounding generally annular area, and a second optical coupling device having an optical input portion generally aligned with the second optical axis to collect the further pump energy and having an optical output portion aligned with the second pumping input portion for transferring the further pump energy to the inner cladding.
From a further broad aspect of the present invention, there is provided an optical fiber pumping method for use with a double-cladding fiber in an optical amplifier or laser configuration, said fiber Including a single-mode core doped with an active material and disposed Within an inner multi-mode cladding surrounded by an outer cladding, said inner cladding being provided with a first pumping input portion located at a first input end of the inner cladding. The method comprises the steps of: i) radiating pump energy parallel to a first optical axis from a plurality of sources disposed in a first spatial arrangement through a generally annular area surrounding the first optical axis while leaving a middle area for one of an input optical signal injection into the core and an output optical signal extraction from the core; ii) collecting the pump energy; and iii) transferring the pump energy to the inner cladding at the first pumping input portion.